This invention relates to a water-soluble photopolymer and method of forming a pattern by using the same, and more particularly to water-soluble photopolymer by use of which the resolution of photolithography is greatly improved to the level of a submicron order, and the method of forming a pattern wherein high resolution of the pattern is accomplished.
The enhancement of the degree of a integration of semiconductor integrated circuit leading up to the level of VLSI owes much to the improvement of resolution of the photolithography by the use of light. This improvement of resolution has been promoted to the limit of the light by the technical development of hard ware, i.e., light exposure apparatus and by technical development of software, i.e., pattern forming method (for instance, the development of multilayer resist technology). At present, lines in the order of microns can be drawn by the use of photolithography, and a 1M-bit DRAM is realized in the MOS memory field. Furthermore, an advanced photolithographic technology for drawing lines of a submicron order has been energetically researched and developed recently, and a hardware approach, such as the improvement of performance of an optical system using a reduced projection exposure method, has been reported. On the other hand, the enhancement of performance of the resolution from the aspect of using a process of pattern forming technology has been also proposed recently. In either case, however, the question "where is the limit of light?" is not clearly answered yet. Incidentally, lithography using radiation other than light, such as X-rays, and electron beam wafer direct writing, is being studied, but considering its technical barrier and existing production facilities, it seems the shortest way to achieve the VLSI to improve the existing lithography using light, is in the direction of extending the "life of light," rather than seeking the technology of a non-light lithography.
To form a high-precision pattern of a submicron order by said photolithographic technique, the first step is to realize a high performance, high resolution resist, the second step is to realize a multilayer resist to present the effects of step differences on an LSI substrate and a reflection on a high reflectivity substrate, and a third step is to realize a film material and pattern forming method possessing the function of substantially enhancing the resolution which was conventionally limited in the known methods.
Conventionally, regarding the first point of realizing a high performance, high resolution resist, resists of nylon, unsaturated polyesters, urethane, polyvinyl alcohol (PVA), cellulose acetate, diazo-novolak and other systems have been disclosed as positive resists, and all but PVA resists are developed in an alkaline solution or an alcohol or other organic solvents. Unlike the others, the PVA resist can be developed in water, which is a great advantage in waste liquid treatment or other aspects. However, the PVA resist is too high in viscosity, and is likely to deteriorate at room temperature, and it is hard to handle. Besides, this high viscosity requires an immensely long time in the mixing with other monomers or photopolymerization starters in the resist pattern forming process. The dissolution speed of PVA in water is not necessarily high, and development in water takes a long time, and since the resin in the hardened part may slightly elute, the pattern image obtained by development tends to be unclear. If the molecular weight is lowered in order to reduce this viscosity, however, the mechanical strength of the obtained resin is significantly lowered. One may consider lowering the viscosity by diluting the aqueous solution at the time of preparation, but this is quite uneconomical because the viscosity is changed due to evaporation of water.
Relating now to the second point of preventing the effects of step difference and reflection, in a desired multilayer resist, wires are formed in steps on a substrate such as semiconductor substrate, and supposing the surface to be coated with one layer of resist, if the film thickness of the resist applied on a flat substrate is assumed to be t.sub.R1, the resist film thickness on the stepped part is t.sub.R2 being determined by the viscosity of the resist itself and the rotating speed at the time of coating. At this time, to set t.sub.R1 =t.sub.R2, that is, to eliminate the difference of resist film thickness at the undulated part is physically impossible. Thus, in a film thickness of t.sub.R1 .noteq.t.sub.R2, when a resist pattern is formed at right angle to the step pattern, the pattern width is determined as 1.sub.1 at the position of film thickness t.sub.R1 of resist pattern, and since there is a relation of t.sub.R1 &gt;t.sub.R2 at the position of film thickness t.sub.R2, the pattern width becomes 1.sub.2 (1.sub.1 &gt; 1.sub.2), and a dimensional conversion difference occurs at the stepped part. That is, in a very delicate pattern, a favorable line width control is not achieved, and the thickness of the edge part of step becomes substantially greater than the film thickness t.sub.R1 of the flat part, the resolution declines. Generally, the resolution increases as the film thickness of resist decreases. This is because the incident energy is attenuated due to an interference and diffraction phenomenon when the gaps are finer by the wavelength of the radiation itself. That is, if only the resist is applied in a thick coat to reduce the apparent resist film thickness difference for the purpose of decreasing the resist film thickness difference on the stepped part, the resolution is lowered, which is not favorable from the viewpoint of pattern forming.
To enhance the resolution and dimensional precision on the step by the single layer resist, conventionally, a tri-layer structure, a double-layer structure, and other multi-layer resist systems have been proposed. The most popular tri-layer structure is described below. A step is formed on a substrate, and a thick coat of organic film, such as photo resist, is applied on it, and an inorganic film, for example, plasma Si.sub.3 N.sub.4 film or spin-on-glass (S.O.G.), is formed on said organic film, then a thin layer of resist is applied as the top coat. By patterning this top coat of resist layer, a resist pattern is obtained. By a dry etching technique through this resist pattern, an inorganic film pattern is obtained. Finally, through the resist pattern and inorganic film pattern, an organic film pattern is formed by an oxygen system gas plasma.
In this pattern forming by a tri-layer resist, the resolution is high because the resist can be applied thinly as the top coat, and since the organic film in the bottom layer is applied thickly, the resist pattern may be obtained without the effect of step differences on the substrate surface, so that the deterioration of dimensional precision is small. However, it is technically difficult to detect the end point in a dry etching technique and the etching condition is complicated because of the multilayer structure, and the processing time is long and it is not preferable from the standpoint of mass production and economy.
To solve these problems of tri-layer resist, a double-layer resist structure is proposed. For example, in the double-layer resist structure, a positive resist, in particular, a positive ultraviolet (UV) resist is explained below. A positive UV resist is applied on a substrate such as semiconductor substrate in a greater thickness than the stepped part of wiring or the like to make the surface flat, and soft baking is applied. By irradiating the entire surface of the positive UV resist with UV light, a sensitized positive UV resist is obtained. This sensitized positive UV resist is heat-treated to such an extent that the photosensitive reaction may not be lowered, and this positive UV layer is treated with a halide solution such as CF.sub.4, CCl.sub.2 F.CClF.sub.2, CCl.sub.4 or SF.sub.6 solution by a spin-on or dip method, and a positive UV resist denatured layer is formed. Next, a second positive UV resist of the same type as a positive UV resist of first layer is applied on the denatured layer of the first positive UV resist, and baked. At this time, since the denatured layer has been formed, the second positive UV resist may be laminated in a separate form, without melting, on the first positive UV resist. Using a mask having a pattern, the first and second positive UV resists are selectively irradiated with ultraviolet rays in other parts than the chrome part which is the light shielding area. The surface is developed and removed, leaving the second positive UV resist and first positive UV resist in other parts than the ultraviolet ray exposed parts. In this series of processes, a fine pattern can be formed while applying the resist layer in a thick coat, and the dimensional precision at the stepped part is improved.
However, it is difficult to laminate resists of identical shape and identical composition, and complicated processes such as surface treatment are increasing.
Concerning the third point of limitation of the resolution due to the optical performance of a exposure device, generally, the resolution R of ultraviolet ray exposure is expressed in the following Rayleigh's law. EQU R=0.6.times..lambda./NA.times.(1+1/m) (1)
where
.lambda.: wavelength PA1 AN: lens numerical aperture PA1 m: magnification
To enhance the resolution, a shortening of the wavelength or an increase of NA may be considered, but at the present technical level of an optical system, if the wavelength is 365 nm (i-line) and NA is 0.4, the resolution R is 0.6 .mu.m, which is estimated to be inferior to that of electron beam exposure or X-ray exposure.
In 1983, however, B. F. Griffing et al. of General Electric Co. of the United States disclosed a method of improving the resolution and pattern shape by laminating the contrast enhancement layer to promote the contrast of the optical strength profile on the resist for pattern forming (Contrast Enhanced Photolithography, B. F. Griffing et al., IEEE-ED, Vol. EDL-4, No. 1, Jan. 1983). According to their report, the resolution is possible up to 0.4 .mu.m in the ordinary reduced projection method (.lambda.: 436 nm, NA: 0.32).
As a result of research by the present inventors, the following requirements are found as the characteristics of the pattern forming organic film for enhancing the contrast.
First of all, relating to the problems in exposure by light, for example, in an ordinary reduced projection method, the optical strength profile of the output by reduced projection ekposure method is processed by its optical lens system. To wit, if exposed through a reticle, the ideal diffraction-free incident light intensity profile is a perfect short waveform, and its contract C is expressed in the following equation. ##EQU1##
At this time, the contrast C is 100%. Its input waveform, when passing through the optical lens, is Fourier-transformed by the transmission function of the optical lens system, and becomes closer to the shape of a cosine wave as the output waveform, and the contact C is deteriorated at the same time. This contrast deterioration greatly affects the pattern formation, for example, the resolution and pattern shape. Incidentally, the contrast required for resist pattern resolution is said to be more than 60% the characteristic of the resist itself, and when the contrast C becomes less than 60%, the pattern formation is disabled.
Using a resist film which tends to be small in the transmission of ultraviolet rays in a region of short exposure time (small exposure energy) and large in a region of large exposure energy, by passing said output waveform close to cosine waveform through this film, it has been found that the contrast C tends to increase. To explain this further quantitatively, the parameter expressed as an exposure absorption dependent term A of the positive resist in a report by F. H. Dill et al. of IBM, U.S.A. (Characterization of Positive Photoresist, F. H. Dill et al., IEEE-ED, vol. ED-22, No. 7, July 1975) is used. Generally, A is expressed as follows. ##EQU2## and it is preferable that A tends to be greater for the enhancement of contrast. For the increasing tendency of A, the smallness of d (film thickness) and the largeness of ratio of T(o) (initial transmission) to T(.infin.) (final transmission) are required.
Returning here to the GE report, below is explained the pattern forming process using the contrast enhanced lithography (CEL) disclosed by Griffing et al. of GE. In the first place, a resist is applied by rotation on the substrate. Next, a contrast enhanced layer (CEL) is applied by rotation on this resist. The surface is selectively exposed with ultraviolet rays (UV) by the reduced projection method. At the same time, part of the resist is selectively exposed, and the entire CEL is removed. Finally, an ordinary developing process is given to from a resist pattern. In this method, the required conditions of CEL are the property to enhance contrast, an ability to prevent melting of the resist for forming the lower layer of pattern, and an ability to prevent deterioration of the characteristic of the resist by the fluid to remove the CEL. Because of these requirements, the material composition is considerably limited. Besides, the CEL removing process exists as a complicated and risky step from the viewpoint of the process operation. Furthermore, on a substrate of a large scale integration circuit (LSI), a step difference is always present, and the surface is not made flat if a resist is applied on the step. When a layer to enhance the contrast, that is, CEL is applied on this uneven resist surface, a difference in film thickness occurs as a natural consequence. That is, the surface is not flat. Since parameter A in eq. (3) depends on d (film thickness), the value of A varies with the fluctuation of film thickness, and the contrast enhancing effect is altered at the same time, and the resist line width changes, which may lead to deterioration of pattern precision. This is further explained. When a resist is applied on a stepped substrate, the film thickness of the resist does not become flat in the present rotary coating method. Next, when a CEL film is applied on the resist, the thickness of the CEL film varies, and said parameter A changes. Therefore, when the CEL film is removed by developing the resist and forming the pattern, the width of resist pattern does not remain constant, and the precision of the pattern width deteriorates.